1. Technical Field
The present invention relates generally to flow mixing methods and devices for jet engines and more particularly to a device and method for mixing flows to attenuate noise and augment thrust.
2. Background Art
Flow mixing devices have attracted significant attention since the invention of the jet engine. One application for these devices is related to the attenuation of the noise that is produced by the exhaust during the operation of the jet engine. Since noise generation is proportional to the velocity of the jet engine taken to the sixth or eighth power depending upon the pressure ratio of the nozzle, reductions in the velocity of the exhaust have the potential to significantly attenuate noise.
Static mixers and steady flow ejectors have been employed to breakup the exhaust flow into smaller jets to enhance the rate with which the exhaust flow is mixed with the ambient flow field. In operation, these devices create large shear areas between the jets and the ambient flow field, causing the streams to interact at their interfaces and exchange shear force and momentum. As this process is irreversible in nature, the operation of these devices substantially reduced the efficiency of the engine. Consequently, it was possible to achieve a similar degree of noise attenuation by simply omitting the flow mixer and reducing the power level of the jet engine.
Flow mixing devices have also been investigated due to their potential to augment the thrust that is produced by the engine. Several problems have been noted, however, when these devices have also been employed to attenuate noise. In the case of steady flow ejectors, the ram drag that results from the drawing of ambient air into the ejector for mixing with the exhaust flow overtakes the level of augmented thrust that is produced when the jet engine is operated at a relatively low speed (i.e., approximately Mach 0.3 to Mach 0.4). As such, it has not been possible to employ these steady flow ejectors in an efficient manner, particularly where the jet engine is to operate at a supersonic speed.
In view of the drawbacks of steady flow ejectors, a second kind of ejector has been proposed to take advantage of the unsteady flow physics. The known unsteady flow ejectors produce a primary jet having a relatively large radial velocity component that must be turned back to the axial direction in order to provide thrust. This turning must be done by the secondary flow at a cost of its axial momentum, thereby generating a substantial loss of the available propulsive energy.
In one preferred form, the present invention provides a jet engine assembly having a turbofan jet engine and an unsteady flow ejector. The turbofan jet engine has an engine core for powering a fan. The engine core produces an engine core flow and the fan producing a fan flow. The unsteady flow ejector has a multi-bladed rotor which is disposed within the engine core flow and rotates in response to a transfer of momentum from the engine core flow. Rotation of the rotor within the engine core flow generates a plurality of high velocity, low density rotating jets and a plurality of low pressure voids, with each of the voids being spaced between two of the jets. Each of the voids entrains a portion of the fan flow which then mixes with the jets and produces a mixed flow having a relatively higher flow rate and a relatively lower velocity than the engine core flow.
In another preferred form, the present invention provides a jet engine assembly having an inlet, a turbojet engine and an unsteady flow ejector. The inlet provides an inlet flow of air to the turbojet engine. The turbojet engine receives at least a portion of the inlet flow, employing it to generate a propulsive primary flow. The unsteady flow ejector includes a multi-bladed rotor that is disposed within the primary flow and rotates in response to a transfer of momentum from the primary flow. The rotor employs the primary flow to generate a plurality of high velocity, low density rotating jets and a plurality of low pressure voids, with each of the voids being spaced between two of the jets and being selectively employable to entrain a secondary flow of air into voids. The jet engine assembly can be operated in a first mode wherein the secondary flow is a flow of ambient air that is introduced directly into the unsteady flow ejector to mix the primary and secondary flows so as to attenuate the noise level of the air exiting the turbojet engine.
In another preferred form, the present invention provides a method for reducing the noise emitted by a flow of exhaust from an outlet of a jet propulsion engine. The method includes the steps of segregating the exhaust flow into a plurality of rotating high velocity, low density jets and a plurality of rotating low pressure voids; and employing the low pressure voids to entrain at least a portion of a secondary flow of air, the jets and the entrained secondary flow mixing to produce a mixed flow having a relatively higher flow rate and a relatively lower velocity than the exhaust flow.